Polylactic acid (referred to herein as “PLA”) is becoming an important industrial chemical because it is a biodegradable plastic that is not derived from petroleum. PLA is a renewable resource that is derived from corn, potatoes, and various plants. PLA is referred to as a carbon circulation-type plastic because it is produced from lactic acid and after use can be broken down to water and carbon dioxide through biodegradation or incineration.
PLA has a mechanical strength at room temperature that is close to that of polyethylene terephthalate (PET), and is easily manipulated. Because of these characteristics, PLA is expected to become a general-purpose plastic material that is commonly used in daily life. PLA does, however, have drawbacks based on its heat resistance, fragility, and low flexibility.
One particular suggested use of PLA is in the area of adhesives, particularly adhesives that are prepared in packages (e.g., packets or containers). For this particular use, however, flexibility is important. Thus, enhancing the flexibility of PLA has received much attention, and numerous methods for improving that characteristic have been proposed.
In one method of improving the flexibility, other aliphatic ester, ether, or carbonate components are introduced into the polylactic acid skeleton by copolymerization in order to impart more flexibility. This method increases the cost of the resulting product because of the nature and amount of the added components.
Another method of improving the flexibility of PLA adds a plasticizer having a low molecular weight (for example, polyethylene glycol) to the PLA. Addition of a plasticizer, however, causes bleeding (separation) of the plasticizer from the surface, which can result in a sticky, tacky surface.
Yet another method of improving the flexibility of PLA blends polyolefins, such as low-density polyethylene (LDPE), with PLA. Because polyolefins and PLA are not compatible with each other, macroscopic phase separation occurs and as a result, the PLA blends became optically opaque. This is not desirable or acceptable for applications where optical transparency is important.
Other methods of improving the flexibility of PLA includes the addition of an acrylic acid ester resin having a relatively low glass transition temperature (Tg) to the PLA, the addition of a second polymer (having a weight average molecular weight of 30,000 g/mole or less) mainly comprising an unsaturated alkyl carboxylate-based unit that has a glass transition temperature of 10° C. or less, or the addition of an acrylic acid ester-based oligomer. These methods typically do not provide a PLA resin with the desired combination of flexibility and elongation properties. Further, in some of the known packaging materials where a second polymeric or oligomeric material is added to the PLA and the packaging materials are held at room temperature for several days, the second polymeric or oligomeric material can separate from the packaging material resulting in a sticky texture that is not useful. Therefore, there remains a need for further methods of enhancing at least one property of a PLA packaging material.